No abstract

We analyze the experimental conditions needed for creating two kinds of dipolar order, namely, intrapair and interpair order in thermotropic liquid crystals. By adapting to the case of liquid crystals the model of weakly coupled spin pairs first developed for oriented hydrated salts, we obtain that the dipolar signal at every preparation time can be regarded as a weighted sum of the pure intra- and pure interpair signals; the weights being determined by the amount of each kind of order resulting from the preparation sequence. The dipolar signal predicted by the model is symmetric in the preparation and observation times and the intrapair component is, in a good approximation, proportional to the time derivative of the FID, regardless of the number of different dipolar couplings (inequivalent pairs) present in the molecule. From this model we obtain a prescription for preparing the different dipolar orders both when the pairs are strictly equivalent or when they are not. The applicability of the spin thermodynamics approach in liquid crystals is tested in two typical thermotropic nematic samples: PAA d(6) (methyl deuterated p -azoxyanisole) and 5CB ( 4(') -pentyl-4-biphenyl-carbonitrile).

Des mesures de RPE sur la lacune d'oxyghne ayant pibgb un blectron (centre I?+) dans ZnO, ont permis de dbterminer les parametres des interactions hyperfines magnetique et quadrupolaire blectrique, avec un noyau ZnS7 adjacent Q la lacune. Ceci dans les deux cas possibles de configuration: celui qui correspond 8. un Zna7 dam un site axial par rapport ? L la lacune, et l'autre, correspondant au noyau dans un des trois sites non-axiaux. A partir des valeurs expbrimentales du paramktre quadrupolaire P I I = (3,13 & 0,Ol) MHz (axial) et (3,l & 0,2) MHz (non-axial) nous montrons que les quatre ions Zn++ premiers voisins, relaxent vers l'extbrieur de la lacune. The oxygen vacancy with one trapped electron (F+ centre) has been studied in ZnO by EPR. The magnetic hyperfine and electric quadrupole interactions with Zna7 nuclei which are nearest neighbours to the vacancy were determined for the two possible configurations, namely a Zn67 nucleus in a) the axial site with respect to the vacancy and b) one of the three non-axial sites. From the experimental values of the quadrupole interaction Pi1 = (3.13 & 0.01) MHz (axial) and (3.1 & 0.2) MHz (non-axial), it is deduced that the four nearest neighbour Zn2+ ions relax outwards from the vacancy.

An approach for nuclear quadrupole spin relaxation by anharmonic lattice vibrations of quadrupole nuclei situated at molecular or ion groups is presented. The temperature dependence at constant crystalline configuration is analyzed. This approach takes advantage of the existence of tightly bound molecules or molecular groups. Fluctuations causing relaxation are considered as collective vibrations of the whole lattice. Anharmonicity is explicitly included by involving cubic and quartic terms of the lattice vibrations Hamiltonian. The spectral densities of the spin-lattice Hamiltonian are expressed in terms of the phonon spectral densities and the temperature Green-function perturbation formalism is used for calculating the lower-order contributions. Three kinds of processes are found to contribute mainly to the spin-lattice relaxation time (T, ), namely, anharmonic Raman (AR), first-order Raman (1R), and a third mechanism originated in the self-energy effects on the two-particle spectral density. The AR and e J )2~2( Fcr( t )Fere ) e (2 dt J J

Starting from the hypothesis that the decay of coherent signals observed in 1H NMR experiments is driven by quantum interference, irreversible decoherence, and nonidealities in the experiment, we design an experiment to isolate and identify the irreversible attenuation of multiple-quantum coherences toward quasiequilibrium states of dipolar order in nematic liquid crystals (LCs). The experiment combines the well-known "magic sandwich" pulse sequence with preparation of dipolar ordered states and encoding of multiple-quantum coherences. The spin system composed of the dipole-coupled protons of a LC molecule provides an example of a small cluster of strongly interacting spins. We study decoherence rates under a sequence that reverses time evolution with the secular dipolar Hamiltonian to compensate coherent evolution of a closed quantum system. In this way, the time scale is made evident where irreversible decoherence takes place, providing insight into the nature of the processes responsible for the attainment of quasiequilibrium. The behavior of single- and double-quantum-coherence amplitudes with reversal time is interpreted as evidence of the quantum character (as opposed to stochastic character) of the processes that drive irreversible decoherence. The experimental method proposed is useful for probing the action of the environment on materials with quantum information processing potential.